The present invention relates an automatic convergence alignment system for a color television display apparatus, and in particular to an automatic convergence alignment system whereby after initial manual adjustment of dynamic and static convergence alignment, static convergence alignment is subsequently adjusted automatically.
The invention is especially applicable to projection display color television receivers. Generally speaking, a projection display type of television receiver utilizes three projection cathode ray tubes (referred to in the following as projection tubes) for generating displays of the three primary colors red, blue and green, in a large-scale picture projected on a display screen. The respective angles of incidence upon this screen of the three beams projected by these tubes will mutually differ to some extent, producing color errors in the displayed picture unless corrected, i.e. unless convergence alignment is executed such as to ensure correct registration between the respective color displays produced by the projection tubes. Such convergence alignment is basically of two types, dynamic and static. With dynamic convergence alignment, respective magnetic fields (generated by means of respective convergence yokes as described hereinafter) are applied to the electron beams produced by the projection tubes, which vary periodically in synchronism with the horizontal and vertical scanning of the beams such as to dynamically apply varying amounts of deflection of each beam such to ensure correct registration of the colors. With static convergence alignment, a fixed preset magnetic field is applied to each of the electron beams, such as to apply fixed amounts of deflection bias to each electron beam along the horizontal and vertical directions of the displayed picture, so as to ensure correct color registration. Usually, adjustment for both static and dynamic convergence alignment is carried out prior to use of such a television receiver, e.g. at the time of manufacture. However such convergence alignment adjustment requires a substantial amount of time and also a certain degree of operator skill, and hence is a disadvantage with regard to reducing manufacturing costs. In order to overcome this disadvantage, it has been proposed in the prior art to incorporate digital memory circuits in the television receiver, and to generate a cross-hatch display on the screen of the receiver, for each of the primary colors, as shown in FIG. 1. When initial convergence alignment has been carried out, then data representing the respective amounts of adjustment required for each of various adjustment points 1 (e.g. the cross-points of the cross-hatch alignment pattern) are stored as digital values in a 1-frame memory. During subsequent operation, the data for each of these adjustment points are read out and subjected to digital-analog conversion, to produce independent values of correction which are applied for each of the adjustment points. This has the advantage of providing very precise convergence alignment, since adjustment is performed independently for each of the adjustment points.
A second method of convergence alignment which has been proposed in the prior art is designed to provide automatic convergence alignment during operation of a projection type color television receiver, and will be described referring to FIG. 2. Here, reference numeral 10 denotes one of the projection tubes of a projection type color television receiver, numeral 11 denotes a projection lens, numeral 13 the display screen, numeral 9 a deflection yoke, numeral 8 a convergence yoke. The projection tube 10 is driven from a video signal applied from an input terminal 2 through a video amplifier 4. Normally, the video amplifier 4 operates in the same way as the video amplifier of a direct-display color television receiver. However during adjustment of convergence alignment, a convergence alignment pattern such as a cross-hatch pattern (i.e. as for the prior art example of FIG. 1) is produced from a digital convergence circuit 5 and supplied to the video amplifier 4 to drive the projection tube 10. The deflection yoke 9 is driven by a deflection circuit 6, which operates in synchronism with horizontal and vertical synchronizing signals applied from an input terminal 3, to control scanning by the electron beam of the projection tube 10. Although as stated previously, such a projection type color television receiver generally incorporates three (R, G and B) projection tubes, only one of these is shown for simplicity of description. A television camera 12a is positioned to receive light from the screen 13, and video signals thus produced from the camera 12a are supplied to an alignment pattern detection circuit 12b, which functions to detect the alignment pattern displayed on the screen 13 as described above. Results of this detection are supplied to an adjustment point detection circuit 7, which detects the degree of convergence attained at each of the adjustment points of the alignment pattern, and accordingly modifies respective alignment compensation quantities which are produced for the respective adjustment points by the digital convergence circuit 7, in accordance with any misconvergence which is detected by the adjustment point detection circuit 7. In this way, automatic convergence alignment adjustment can be rapidly executed.
Another prior art method which has been proposed for executing automatic convergence alignment adjustment is basically similar to that of FIG. 2 described above, but utilizes a special screen having an array of photo-detector elements for detecting misconvergence. Description of this method will be omitted.
Each of the prior art convergence alignment methods described above can provide highly accurate convergence alignment. The second method, of FIG. 2, has the further advantage of being automatic. However in a practical projection type color television receiver, some mis-convergence will be produced immediately after power is switched on, and the amount of mis-convergence will thereafter gradually vary as the operating temperature of the receiver varies. In addition, drift of component characteristics over a period of use, changes in the neck charge of the projection tubes, drift in the DC level of the output signals from the digital convergence circuit, changes in shape of the convergence yoke due to the effects of heat, etc., will also combine to produce some mis-convergence. Such mis-convergence can in general be corrected by static convergence alignment adjustment (as defined hereinabove), and so will be referred to as static mis-convergence in the following. Thus, to ensure a reasonable degree of convergence alignment when such a prior art method of convergence alignment adjustment is used, it is necessary to carry out aging of the television receiver over a substantial period of time.
In addition, in the second prior art method described above in which a camera is used in detecting amounts of mis-convergence, it is necessary to perform complex signal processing and to utilize large-scale circuits, while moreover it is necessary to employ a substantially expensive video camera. In addition, it is not possible to view a normal television picture while such automatic convergence alignment is in progress, since the alignment pattern extends over the display screen.
In the case of the prior art method in which photo-detector elements are mounted directly on the screen, for detection of mis-convergence, the problem arises that it is necessary to utilize an auxiliary screen for executing convergence alignment adjustment and to use a main screen for the usual television display. Alternatively, it is necessary to provide a special screen which is a combination of such a main screen and auxiliary screen. Furthermore in the case of a front-projection type of display, the problem arises that errors will occur due to the effects of unwanted light falling on the photodetector elements from external sources.